1. Field of the Invention
The present invention relates to a digital photographing device, and in particular, to a digital photographing device which photographs a subject via an optical lens and acquires digital image data expressing the subject.
2. Description of the Related Art
In a photographing device which photographs a subject through an optical lens and acquires an image which shows the subject, distortion arises at the periphery of the acquired image due to refraction of the lens. Such distortion is generally called optical distortion, and is classified into the following two types: “pin-cushion distortion” in which the corner portions of the image extend outwardly, and “barrel distortion” in which, conversely, the corner portions contract. It is generally known that, in both cases, the amount of distortion (amount of displacement) is determined in accordance with the distance from the optical center.
In the case of a photographing device in which an obtained image is recorded on a film (such as in the case of a silver salt camera), it is not possible to correct an image after recording, and the optical distortion is determined by the lens performance. On the other hand, in the case of a photographing device (such as a digital still camera or a digital video camera or the like) in which an image is acquired by digital data and recorded on a recording medium (such a device is called a “digital photographing device” hereinafter), even after recording, the image can be corrected by computational processing. Thus, in the field of digital photographing devices, techniques relating to the correction of optical distortion have conventionally been proposed.
For example, Japanese Patent Application Laid-Open (JP-A) No. 6-292207 discloses a method in which a correction amount is determined at each coordinate of an image, and the correction amounts are stored in advance as a table in a memory. On the basis of the stored correction amounts, linear interpolation is carried out on digital image data obtained by photographing a subject, such that each pixel is corrected. However, in this technique, the memory which stores the table of correction amounts requires a storage capacity corresponding to the size of the image. If the image size is large, the storage capacity required by the memory is also large. This results in a reduction in the work region and an increase in costs.
A technique intended to solve this problem is the method, disclosed in JP-A No. 11-252431, of carrying out correction by expressing the correction amounts by an approximate expression. Namely, it is generally known that the displacement amount of the optical distortion can be approximately expressed by a polynomial, and the inverse thereof is used as a correction formula. In this technique, there is no need to hold the correction amounts of the respective coordinates in a table. Provided that only parameters (the coefficients of the polynomial) are stored in a memory, all of the coordinates of an image before and after correction can be set in correspondence by computation. Further, in order to handle different focal lengths such as the telephoto end and the wide-angle end and the like, JP-A No. 11-275391 discloses storing parameters for each focal length, and carrying out correction by selecting the parameters in accordance with the focal length at the time of photographing.
Moreover, JP-A No. 11-250240 discloses a technique of making the correction processing faster by converting digital image data (RGB data) to YUV data, thinning out the UV data thereamong, and carrying out correction.
Note that, among application software executed at computers (PCs), there are application software for carrying out correction processing of optical distortion. Image data, which was photographed by a digital photographing device, is transmitted to a PC, and the optical distortion can be corrected by the PC executing the application software. However, the work for installing in the PC the application software which carries out the optical distortion correction processing, and the work for transmitting the digital image data for which correction is desired to a PC (a built-in RAM or the like), are required. In order to decrease the burden of work borne by the photographer, it is desirable to carry out correction at the digital photographing device as in the above-described techniques.
A digital photographing device generally has a display portion such as an LCD (liquid crystal display) for displaying the image (a still image or dynamic images) which shows the subject. It is often the case that the subject which is currently being photographed is displayed as dynamic images on the display portion substantially in real time (so-called through image display). The display portion can thereby be used as a finder.
However, in all of the above-described conventional techniques, correction is carried out by the same correction section regardless of the type of digital image data. Thus, for example, correction at a high correction accuracy of the level used for still images is carried out as well on dynamic images which do not require high correction accuracy. Thus, a problem arises in that processing requires more time than needed.
In particular, conventional techniques cannot handle cases in which dynamic images showing a subject are displayed substantially in real time on the display portion, and can only display dynamic images without carrying out correction thereon. Thus, even if the photographer carries out photographing by confirming the subject on the finder, the subject, which is expressed by the digital image data obtained by the actual photographing, is different from that which the photographer imagined, and there are cases in which photographing must be carried out again.